Method for determining a copy to be decoded and an associated erasures vector, corresponding storage means and receiver device

ABSTRACT

A method is proposed for determining an erasures vector associated with a data block to be decoded built out of received copies, and using levels of reliability of transmission associated with symbols contained in different copies of a same block of received pieces of data. 
     Such a method advantageously makes reduces the error rate at output of a decoder by minimizing the operations to be performed to determine the erasures vector, marking the erasures and preventing the generation of an excessively large number of erasures (even when the transmission of certain received copies is unreliable) to have a number of erasures that does not exceed the correction capacity of the decoder.

1. FIELD OF THE DISCLOSURE

The field of the disclosure is that of communications networks. Thedisclosure can be applied especially but not exclusively in a wirelessnetwork.

More specifically, an embodiment of the invention pertains to thedetermining of a copy to be decoded and an associated erasures vector bya receiver device receiving, through a communications network, severalcopies of a same original piece of data encoded according to an errorcorrection code.

2. TECHNOLOGICAL BACKGROUND

We shall strive more particularly here below in this document todescribe a set of problems existing in the particular case of “mmWaveWPAN (Millimeter Wave Wireless Personal Area Network) type wirelessnetworks.

The disclosure naturally is not limited to this particular field ofapplication but is of interest for any decoding technique that needs tocope with a proximate or similar set of problems.

The mmWave WPAN type networks mentioned here above are particularly wellsuited to applications that are heavy consumers of bandwidth, forexample home audio-visual applications. Indeed, the 60 GHz radio bandused by this type of network offers a substantial bandwidth, thusenabling the transportation of large quantities of data as is requiredfor example for the transportation of video streams with 1080presolution. However, frequencies in the range of 60 GHz have wavelengthsin the range of one millimeter and are easily absorbed by obstacles(walls, furniture, living beings etc) because of the very low rate ofreflection on obstacles and a very high rate of attenuation in air.

In a communications network, communications between a sender device(here below also called a source node) and a receiver device (alsocalled a destination node here below) can be easily disturbed and datacan be lost.

To resolve this problem, two approaches are commonly used. They consistin implementing either a temporal redundancy with transmission atdifferent points in time or spatial redundancy through a use of severalantennas and modules for processing a physical signal. These twoapproaches can be combined by use of a meshed network in which datarelays are set up, according to a predefined clocking rate. Thediversity of the relays provides spatial diversity and the controlledclocking (in the form of TDMA or Time Division Multiple Access typeaccess) provides temporal diversity. In this type of network, the datatransmitted by the source node can therefore flow on different paths.Several copies (there are therefore redundancies) of the same pieces oftransmitted data can therefore be received by the destination node andthus facilitate the correction of errors in order to retrieve dataoriginally sent out by the source node.

Spatial redundancy is facilitated by the small wavelength of the radiosignals which enables the making of miniature antennas. It is thenpossible to envisage the installation of several antennas and modules(or blocks) for receiving signals for a same receiver device. Thus, thesignals reach each of the antennas of the receiver device alongdifferent paths and the risks of having a concealment of the set ofcommunications paths between the sender device and the multiplereception modules is greatly diminished. To increase the spatialdiversity, it is appropriate to move the antennas away from each other(and therefore to move them away from a common data processing module).To limit the excess cost entailed by this duplication of receptionmodules, it is appropriate to limit the redundancy of the processingmodules to those dedicated to processing of the physical signalscorresponding to the layer 1 of the Open Systems Interconnection model(or OSI model).

These receiver devices must ensure the combination of the datatransmitted by the different modules for processing physical signals soas to profitably use the information redundancy (flowing from thereception of several copies of a same piece of original data).Furthermore, it is necessary to minimize information exchanges betweenthe modules (modules for processing physical signals and commondata-processing module) which have been moved away to provide forspatial diversity. Indeed, in video type applications, the bit rate ofthe data to be processed is very high and it is not desirable toincrease this bit rate greatly by adding information extracted from themodules for processing the physical signals.

In order to overcome data transmission errors inherent in thetransmission channel, it is common practice to use error correctioncodes (or Forward Error Correction or FEC codes) in communicationssystems.

One of the error correction codes most commonly used in wirelessnetworks is the Reed Solomon (RS) code. This code can be defined as theyC(n, k) code where n represents the total number of symbols of the codeword after encoding and k is a number of original data symbols (that aretherefore n-k redundancy symbols).

In the Reed Solomon code, a symbol is a set of m bits. A value oftenused in hardware configurations is m=8.

The Reed Solomon code has a correction capacity of (n−k)/2 errors or(n−k) erasures, one erasure being an error in a symbol for which theposition is known. More generally, the correction capacity C of such acode is defined as follows:C=2×number of errors+number of erasures,the upper limit of C being “d”, the minimum distance of the code withd=(n−k)+1.

In order to reach the borderline correction capacity of a Reed Solomondecoder, it is necessary the mark the position of the erasures. Severalmethods are already well known for this purpose.

For example, one method applicable in the case of a system usingmultiple copies of a same code word (where the multiple copies resultfor example from a multiple reading or a multiple reception) isdescribed in the U.S. Pat. No. 6,389,573. This document proposes to markthe positions of erasures by making a symbol-to-symbol comparison of thedifferent copies of the same code word. The positions of erasures aremarked at the places where the symbols are different from one copy tothe other. For each position other than the positions of erasures, thecopy to be decoded (to be transmitted to the decoder with the erasuresvector) comprises the symbol which is identical in the different copies.The decoding capacity of this method is limited to the capacity of thedecoder, despite the contribution of redundancy made by numerous copiesof a same code word.

It must further be noted that most present-day data communicationssystems implement a combination of error correction systems by combiningseveral error correction codes. One well-known example pertains to theuse of a Reed Solomon type code and a convoluted code as illustrated inFIG. 1 (described in detail here below). Indeed, the concatenation of ablock code processing symbols (sets of bits) and a convolutive typeencoder processing bits gives a high error-correction capacity. Atreception, the decoding of the data generated according to theconvolutive code is conventionally done by a decoder based on theViterbi algorithm.

The Viterbi algorithm selects, in a lattice diagram (a diagramrepresenting possible transitions between the different states of theencoder) a maximum likelihood path (corresponding to the path possessinglevel of reliability that is the highest. This decoder is especially inA. J. Viterbi, “Error bounds for convolutional codes and anasymptotically optimum decoding algorithm”, IEEE Trans. IT., Vol 13(1967), pp. 260-269). For a brief reminder of convoluted codes and theirdecoding by the Viterbi algorithm, reference could also be made to J. K.Omura, “On the Viterbi Algorithm”, IEEE Trans. IT., January 1979, pp.177-179.

Several variance of the Viterbi algorithm have been proposed in order tohave symbol-by-symbol information available on the reliability of thedecoding done. Such variance have been proposed especially in thefollowing publications:

G. Battail, “Weighting of the symbols decoded by the Viterbi algorithm,Ann. Télécomm., Vol. 42 (1987), No 1-2, pp. 31-38);

J. Hagenauer, P. Hoeher, “A Viterbi algorithm with soft-decision outputand its applications”, IEEE Globecom, Dallas, 1989, pp. 47.1.1-47.1.7).

R. Sfez, G. Battail, “A weighted output variant of the Viterbi algorithmfor concatenated schemes with convolutional internal encoding, Eurocode90, Udine, Italy;

Patent document EP0758167.

These algorithms are used to obtain information on reliability of thedecoded piece of data and are more commonly known as algorithms of theSOVA (Soft Output Viterbi Algorithm) type.

In a transmission system implementing spatial and/or temporal redundancyof data, i.e. a system in which multiple copies of the same encodedpiece of data are received in order to minimize the errors oftransmission on the transmission channel (or facilitate itsidentification), the problem is to determine which are the symbols andthe erasures vector to be transmitted to a decoder (for example of theReed Solomon type) to have a minimum error rate at output of decoding.

This problem is particularly preoccupying in the case of a transmissionof data where the data bit rates to be transmitted are very high (in thecase for example of a transmission of high-definition digital videodata) and where it is therefore impossible or penalizing to adding agreat deal of additional information to the data in order to obtain thedesired result at output of decoding, i.e. a bit error rate that is aslow as possible.

Indeed, in this case, the databus connecting the different layers (i.e.the different modules) of the receiver device must convey a high bitrate of information (several Gbps) and it is not desirable to increasethis bit rate greatly.

The US patent document 2007/0115801 describes:

A first method for selecting signals received by the multiple antennas(or reception systems) in order to improve the quality of reception ofthe signal. This first method is based on the use of a parameterrepresenting the quality received on each sequence such as for examplean RSSI (Received Signal Strength Indication) measurement or else againan SNR (signal-to-noise ratio) measurement.

a second method for combining FFT (Fast Fourier Transform) block outputsignals of each reception system. The second method is based on the MRC(Maximum Ratio Combining) method in which a linear combination is madeof two antenna signals so as to obtain a resulting signal that maximizesthe reception level before applying the demodulation. The resultingsignal coming from the MRC module is then demodulated and decoded bymeans of a turbodecoder.

This patent document describes a first method for selecting signals anda second method for combining signals which are situated upline from thedemodulation. These methods are based on a level of power in receptionto decide on the signal to be decoded; thus, the selection is madeupstream from the decoder. However, a signal having a high level ofpower at reception is not necessarily the one that will ensure the besterror rate at output of the decoder.

The U.S. Pat. No. 6,065,149 describes an error correction system used toperform a very precise decoding operation and obtain a low bit errorrate. The internal decoder or inner decoder is a SOVA type. The systemis based on the use of a CRC (Cyclic Redundancy Check) algorithm whichgives information on reliability that is additional to the one given bythe internal decoder. These two pieces of information are used todetermine the erasures vector at input of the external decoder.

The U.S. Pat. No. 7,228,489 describes an encoding system that combines aViterbi decoder and a Reed Solomon decoder so as to improve theperformance of the error correction system. The internal decoder is ofthe SOVA type and the information on reliability that it gives issummated by a symbol so as to determine the reliability of the symbol.The symbols that are most probably erroroneous are indicated in theerasures vector of the Reed Solomon decoder.

The U.S. Pat. Nos. 6,065,149 and 7,228,489 mentioned here abovetherefore each propose a particular implementation of a communicationssystem combining two error correction codes (inner code and outer code)but none is located in the particular context where the communicationssystem implements spatial redundancy and/or temporal redundancy(multi-copy system). It is all the more the case that neither of thesetwo documents proposes a permanent solution, in this particular context,to the problem of minimizing the error rates at output of the outerdecoding.

3. GOALS OF THE DISCLOSURE

At least one embodiment of the invention is aimed especially atovercoming these different drawbacks of the prior art.

More specifically, it is a goal of at least one embodiment of theinvention to provide a technique making it possible, in a transmissionsystem implementing spatial redundancy and/or temporal redundancy of thedata, for a receiver device to have a minimum bit error rate at outputof the decoder (first type decoder).

It is another goal of at least one embodiment of the invention toprovide a technique of this kind which, when the decoder of a first typeis used in combination with at least one decoder of the second type,provides information on reliability of the decoded symbols (according tothe second type of decoding) to limit the volume of exchanges within thereceiver device between a first processing block comprising the firstdecoder and a second processing block comprising one or more decoders ofa second type.

It is another goal of at least one embodiment of the invention toprovide a technique of this kind that is simple to implement and costslittle.

4. SUMMARY

One particular embodiment of the invention proposes a method fordetermining a copy to be decoded and an associated erasures vector bymeans of a receiver device receiving at least two copies of a same pieceof data encoded according to an error correction code, each receivedcopy comprising several symbols, each associated with a position indexin said copy, said copy to be decoded being designed to be decoded intaking account of the erasures vector by means of a decoder of a firsttype applying a decoding adapted to the error correction code.

This method is remarkable in that it comprises steps for:

-   -   obtaining, for at least one symbol of each copy received, a        level of reliability of transmission associated with said        symbol;    -   marking or not marking, for at least one given position index        associated with a symbol included in said copy to be decoded, an        erasure for said given position index in said erasures vector as        a function of the reliability levels associated with the symbols        which are positioned at said given position index in said        received copies.

Thus, this particular embodiment of the invention relies on a whollynovel and inventive approach to determining the erasure svectorassociated with a data block to be decoded (which as described in detailhere below is built out of the received copies) by using levels ofreliability of transmission associated with symbols contained indifferent copies of a same block of received pieces of data.

Thus, an embodiment of the invention avoids having to make symbol/symbolcomparisons on all the copies received to mark the positions oferasures, thus minimizing the operations to be performed to determinethe erasures vector.

Furthermore, since it takes account of levels of transmissionreliability, the marking of the erasures is optimized since there is anincrease in the probability that only “true” erasures will be marked assuch. This avoids the generating of an excessively large number oferasures (even when the transmission of certain received copies isunreliable) and therefore gives a number of erasures that does notexceed the correction capacity of the first decoder. Ultimatelytherefore, the error rate at output of the first decoder (for example aReed Solomon type block decoder) is reduced.

Advantageously, this method comprises steps for:

-   -   obtaining, for each received copy, a set of a number N of        position indices associated with N symbol(s) of said received        copy for which the associated level or levels of reliability        is/are the N lowest level or levels from amongst the levels of        reliability of all the symbols of each copy received, with N        being equal to or greater than 1;    -   for at least one given position index corresponding to a symbol        included in said copy to be decoded, marking an erasure in said        erasures vector if said given position index belongs to all the        sets of N position indices obtained for all the received copies.

Thus, an erasure is marked for a given position index only if thereliability of the symbol placed at this index is low for all thecopies. This criterion of decision is simple to implement and requiresfew computations.

Advantageously, the number N of position indices of each set is afunction of a maximum information bit rate that can be accepted by aninterface from:

-   -   a first processing block comprising a decoder of a second type        generating said levels of reliability,    -   a second processing block including said first type of decoder.

Indeed, the greater the value of N, the better the decoding performance(i.e. with the lowest error rate) to the detriment however of asupplementary information bit rate (relative to the levels ofreliability to be transmitted). Thus the decoding performance can beoptimized as a function of the communications capacities between theprocessing blocks.

According to one advantageous characteristic, the method comprises stepswhich, for a symbol positioned at a given position index in said copy tobe decoded, are steps for:

-   -   determining a set of remaining symbols such that, for each copy        received, the symbol positioned at the given position index in        said received copy belongs to said set if said given position        index does not belong to the set of N position indices obtained        for said received copy;    -   checking to see if there is a majority of remaining symbols        possessing a same value;    -   in the event of a positive check, choosing said same value for        said symbol positioned at said given position index in said copy        to be decoded.

Indeed, since the probability of having the same errors on each of thecopies is very low, the same value possessed by said majority ofremaining symbols is deemed to be a reliable value.

According to an advantageous characteristic, the method comprises steps:

-   -   in the event of negative check:        -   for obtaining, for each copy received whose symbol            positioned at said given position index is a remaining            symbol, a level of overall reliability associated with said            copy received;        -   for choosing, for each symbol positioned at said given            position index in said copy to be decoded, the value of the            remaining symbol which is included in the received copy            associated with the greatest level of overall reliability            obtained.

Thus, the probability of not introducing error into the copy to bedecoded is increased through the selection of the copy having the bestlevel of overall reliability.

According to one advantageous characteristic, said receiver devicereceiving said first and second copies, the method comprises stepswhich, for at least one given index position, corresponding to a symbolincluded in said copy to be decoded, are steps for:

-   -   selecting, for said position symbol positioned at said given        position index in said copy to be decoded, the value of the        symbol at said given position index in the second copy received        if said given position index belongs to a first set of N        position indices obtained for said first copy received;    -   selecting, for said symbol positioned at said given position        index in said copy to be decoded, the value of the symbol at        said given position index in the first copy received if said        given position index belongs to a second set of N position        indices obtained for said second copy received.

Thus, in the particular case of a reception of only two copies, theprobability of not introducing error into the copies to be decoded isincreased by selecting a symbol of a higher level of reliability.

According to another advantageous characteristic, the method comprises astep which, if said given position index belongs to no set amongst saidfirst and second sets of N position indices, and if the symbols in thefirst and second copies received possess a same value, is a step forchoosing said same value.

Thus, the probability of not introducing error into the copy to bedecoded is increased by selecting a symbol for which the received copiesare matched.

According to one advantageous characteristic, the method furthermorecomprises steps which, if said given position index belongs to no setamongst said first and second sets of N position indices, are steps for:

-   -   obtaining, for each received copy whose symbol positioned at        said given position index is a remaining symbol such that the        given position index does not belong to the set of N position        indices obtained for each received copy, a level of overall        reliability associated with said received copy;    -   choosing, for said symbol positioned at said given position        index in said copy to be decoded, the value of the remaining        symbol which is included in the received copy associated with        the greatest level of overall reliability obtained.

Thus, the probability of not introducing error into the copy to bedecoded is increased by selecting the copy having the best level ofoverall reliability.

According to one advantageous characteristic, the method comprises stepsfor:

-   -   checking to see whether a total number of one or more erasures        marked in said erasures vector is greater than a maximum        predetermined number S;        and in the event of a positive verification:    -   re-setting the erasures vector in eliminating the pre-marked        erasures;        building a list of position indices for which an erasure has        been pre-marked in the erasures vector;    -   associating, with each position index of the list, the highest        level of reliability amongst those associated with the symbols        positioned at said position index in the received copies;    -   classifying the list by rising order of levels of reliability        associated with the position indices;    -   marking an erasure for the S first position indices of the        classified list;    -   choosing, for each other position index of the classified list        not marked as an erasure, for the symbol positioned at said        position index in said copy to be decoded, the value of the        symbol for which the level of reliability is the highest amongst        the levels of reliability of the symbols positioned at said        position index in the received copies.

Thus, the probability of not introducing error into the copy to bedecoded is increased by selecting the symbols having the best levels ofreliability, and doing so in ensuring that the number of erasuresdetermined for the copy to be decoded remains smaller than or equal tothe maximum number of predetermined erasures S.

According to one advantageous characteristic, said predetermined maximumnumber S of erasures is smaller than or equal to a limit capacity ofcorrection of said error correction code.

Thus, an embodiment of the invention avoids bringing the first decoderto the limit of its correction capacity, solely with the erasures.Indeed, since the value of the parameter S is adjustable, it is possibleto keep a complementary margin of error for the correction of errors(other than the erasures) by the first decoder, and this thereforeminimizes the error rate at output of the first decoder.

Advantageously, the decoder of the first type is a block decoder and thedecoder or decoders of the second type i/are a convolutive decoder orconvolutive decoders.

Thus, an embodiment of the invention can be applied especially when thedecoding done by the first type of decoder is an outer decoding that iscombined with an inner decoding performed by one or more decoders of asecond type, this inner decoding providing information on reliability ofthe inner decoding of each symbol.

Another embodiment of the invention concerns a computer program productthat comprises program code instructions for implementing theabove-mentioned method (in any of its different embodiments) when saidprogram is executed on a computer.

Another particular embodiment of the invention concerns acomputer-readable storage means storing a computer program comprising aset of instructions executable by a computer for implementing theabove-mentioned method (in any of its different embodiments).

Another embodiment of the invention concerns a receiver device adaptedto determining a copy to be decoded and an associated erasures vector,said receiver device receiving at least two copies of a same piece ofdata encoded according to an error correction code, each received copycomprising several symbols, each associated with a position index insaid copy, said copy to be decoded being designed to be decoded intaking account of the erasures vector by means of a decoder of a firsttype applying a decoding adapted to said error correction code.

Remarkably, the receiver device comprises:

-   -   means for obtaining, for at least one symbol of each copy        received, a level of reliability of transmission associated with        said symbol;    -   means for marking, for at least one given position index        associated with a symbol included in said copy to be decoded, an        erasure for said given position index in said erasures vector as        a function of the reliability levels associated with the symbols        which are positioned at said given position index in said        received copies.

5. BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of embodiments of the invention shallappear from the following description, given by way of an indicative andnon-restrictive example, and from the appended drawings, of which:

FIG. 1 is a schematic illustration of a known technique of concatenation(or combination) of error correction codes used for example in satellitecommunications systems;

FIG. 2 is a schematic illustration of an architecture of a transmissionsystem based on a temporal redundancy of data according to a firstparticular embodiment of the invention;

FIG. 3 is a schematic illustration of an architecture of a transmissionsystem based on a spatial redundancy of data according to a secondparticular embodiment of the invention;

FIG. 4 is a flowchart of an algorithm for generating a reliability tableimplemented in a module for receiving physical signals included in thereceiver device of FIG. 2 or FIG. 3, according to a particularembodiment of the invention;

FIG. 5 is an example of a reliability table generated by application ofthe flowchart of FIG. 4;

FIGS. 6 to 8 present a overall flowchart of one particular embodiment ofthe method according to the disclosure:

FIG. 6 is a first part of this overall flowchart in the general casewhere the number of copies received is greater than or equal to 2;

FIG. 7 is a variant of the above-mentioned first part in the particularcase where the number of copies received is equal to 2;

FIG. 8 is a second part of this flowchart enabling especially managementof the number of erasures;

FIGS. 9 a, 9 b and 9 c illustrate an example of application of theoverall flowchart of FIGS. 6 to 8 in the case of a two-copy system andwhen there is little correlation between the two copies;

FIGS. 10 a, 10 b, 10 c and 10 d illustrate an example of application ofthe overall flowchart of FIGS. 6 to 8 in the case of a two-copy systemand when there is a great deal of correlation between the two copies.

6. DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 is a schematic illustration of a known technique for combiningerror correction codes used for example in satellite communicationssystems.

This technique implements a convolutive encoder (more generally alsocalled an “inner encoder”) and a block encoder (more generally alsocalled an outer encoder) for example of the Reed Solomon type.

In a sender device 100, data to be transmitted (grouped together inm-bit symbols according to the Reed Solomon code chosen) are first ofall provided to a Reed Solomon encoder 110 which adds redundancy datathereto.

Then, the pieces of data output from the Reed Solomon decoder 110 (i.e.the data to be transmitted and the redundancy data) are transmitted toan interleaver 120 which mixes these pieces of data together. The mixeddata then passes into a convolutive encoder 130 which adds newredundancy data to them before transmitting them to a modulator 140.This modulator 140 then modulates the pieces of data output from theconvolutive encoder 130 so as to be able to send them on a radio link150 to a receiver device 195.

In the receiver device 195, the signals received through the radio link150 are de-modulated by a demodulator 160 which then transfers datawhich may be vitiated by error to a Viterbi decoder 170. This decoderuses the redundancy data introduced by the convolutive encoder 130 todetect and correct possible errors.

At output of the Viterbi decoder 170, pieces of corrected data 171 arethen transmitted to a de-interleaver 180 which carries out a mixingoperation which is the reverse to the operation performed by theinterleaver 120 of the sender device 110.

The pieces of data 181 coming from the de-interleaver 180 are thengrouped together as Reed Solomon symbols and then applied to the inputof a Reed Solomon decoder 190. This Reed Solomon decoder 190 uses theredundancy data introduced by the Reed Solomon encoder 110 of the senderdevice 100 to detect and correct possible residual errors.

If the Viterbi decoder 170 used implements a SOVA type decodingalgorithm and if the decoding algorithm of the Reed Solomon decoder 190makes it possible to take account of the information on reliability ofthe incoming data, then pieces of auxiliary data 172 representing a bitreliability (or likelihood) of the data decoded by the Viterbi decoder170 are also de-interleaved by the de-interleaver 180. These pieces ofauxiliary data are then converted into reliability information 182 of aReed Solomon symbol type, and then transmitted to the Reed Solomondecoder 190. Each piece of reliability information has an associatedlevel of reliability.

The conversion of the information on ‘bit’ reliability 172 coming fromthe Viterbi decoder 170 into information on “Reed Solomon symbol”reliability 182 is done through bit reliability information on the mbits of the symbol or by choosing the weakest bit reliabilityinformation (i.e. the information for which the associated level ofreliability is the lowest) or again by using any other algorithm forconverting reliability information known to those skilled in the art.

FIG. 2 is a schematic illustration of an architecture of a transmissionsystem based on a temporal redundancy of the data according to a firstparticular embodiment of the invention.

Here below in the description, the term “module” is understood to mean adata processing block.

A sender block 200 consists of two main modules: a MAC TX module 220 anda PHY TX module 210.

The pieces of data to be transmitted are given by an application layerof the sender device 100 to an input/output interface block 225 of theMAC TX module 220, which then transmits them to a MAC processor module233. The MAC processor module 233 performs all the operations forimplementing a transmission medium access control protocol or MACprotocol.

The pieces of data are then transmitted to a Reed Solomon type blockencoder 232 which, after the addition of redundancy data, transmits themto an interleaver 231. This interleaver 231 then mixes the encoded databy permutation of bits in order to improve the robustness relative toerrors of the error correction system.

The pieces of data are then transferred to a PHY/MAC interface 224 whichtransmits them to the PHY TX module 210 by the link 240.

The PHY TX module 210 is in charge of performing operations to transferdata on the radio link as well as an encoding operation to minimize thebit error rate of the communication.

This PHY TX module 210 includes a PHY processor module 215 used toprocess data and convert into a baseband signal.

It also includes an RF radio module 216 which transposes this basebandsignal into a radio signal and then transmits it to a sending antenna217.

The PHY processor module 215 comprises chiefly a convolutive encodermodule 211 which again adds redundancies to the data to be transmittedand a modulator 212 which converts the data into a signal transmittableby radio to a receiver device 250.

The receiver device 250 consists of two main modules: a MAC RX module270 and a PHY RX module 260.

The radio signal coming from the sender device 200 is received by anantenna 267 of the receiver device 250 and then processed in the PHY RXmodule 260. This processing is done by an RF radio module 266 whichtransposes it into a baseband signal that is then transmitted to a PHYprocessor module 265.

This PHY processor module 265 comprises chiefly a demodulator module 262which converts the baseband signal into data and a SOVA type Viterbidecoder 261 which makes a first error correction by using the pieces ofredundancy information introduced by the convolutive encoder 211 of thesender device 200.

The Viterbi decoder 261 provides pieces of corrected data with, for eachof them, a piece of information on reliability. These pieces ofcorrected data are provided to a multi-copy module PKY 268 which appliesan algorithm for preparing a reliability table described in greaterdetail here below with reference to FIG. 4. This algorithm is usedchiefly to adapt the data stream coming from the Viterbi decoder 261 tothe capacity of the communications link 271, 272 between the PHY RXmodule 260 and the MAC RX module 270, this link providing for thetransport of the pieces of corrected data and of the associatedreliability tables. The PHY RX module 260 therefore gives the MAC RXmodule 270, for each copy received, all the pieces of decoded dataaccompanied by a certain number of pieces of information on reliability.

Should the redundancy of the data be of a temporal type (with successivereception of several copies of the same piece of data), the PHY RXmodule 260 will successively give the MAC RX module 270, for example inthe case of two successive receptions, corrected pieces of data D′accompanied by reliability tables Rel′, and then corrected pieces ofdata D″ accompanied by reliability tables Rel″. The corrected pieces ofdata D′ and D″ are transmitted on a communications link 271 and thereliability tables Rel′ and Rel″ are transmitted on the communicationslink 272.

The communications link 271 and 272 as well as the communications link240 may consist of a communications interface, of a serial type forexample, enabling the PHY RX module 260 or PHY TX module 210 to beremoved to a distance by means of a cable. These communications links,in another embodiment, may consist of on-board communicationsinterfaces, of a parallel type for example. These links can furthermorebe shared with inter-module communications other than those describedhere below, thus limiting the available bandwidth available on thesecommunications links for communications between the sender device 200and the receiver device 250.

The pieces of data and information are received in the MAC RX module 270by a PHY/MAC input/output interface block 274 which takes charge ofcommunication between the MAC RX module 270 and the PHY RX module 260.Once the pieces of data are put back in a format enabling the processingin the MAC RX module 270, these pieces of data are transmitted to ade-interleaver 281 which performs the operation in reverse to the oneperformed at sending by the interleaver 231. It must be noted that thede-interleaver module 281 also de-interleaves the information on thereliability table accompanying the data coming from the Viterbi decoder261.

The pieces of data and information are then transmitted to a MACmulticopy module 282 which executes an algorithm to determine the ReedSolomon symbols (forming a copy to be decoded) and an associatederasures vector, to be transmitted to a Reed Solomon decoder 283. Thisalgorithm is described in greater detail here below with reference toFIGS. 6 to 8.

The Reed Solomon decoder 283 then extracts errors if any from the datato be recovered and corrected by using the properties of the ReedSolomon code chosen. Then, it transmits them to a MAC processor 284 tocarry out all the operations of implementation of the MAC protocol usedin the system. The pieces of data coming from the MAC protocol 280 arethen transmitted to the applications layer of the receiver device 250.

Preferably, the MAC TX processor module 220 and MAC RX processor module270 also integrate a random-access memory (RAM) 222, which functionslike a main memory and a computing unit 221 (or CPU i.e. CentralProcessor Unit). The capacity of this random-access memory 222 can beextended by an optional random-access memory connected to an expansionport not shown in FIG. 2. The CPU 221 is capable of executing aninstruction when the communications device is powered on (sender device200 or receiver device 250) on the basis of a read-only memory 223 (orROM). After the CPU unit 221 has been powered on, it is capable ofexecuting the instructions of the RAM 222 pertaining to a computerprogram once these instructions are loaded from the ROM 223 or anexternal memory (not illustrated in FIG. 2). Such a computer program, ifexecuted by the CPU unit 221, prompts a part or all of the steps of thealgorithm described here below with reference to FIGS. 4 and 6 to 8.

It is important to note that the present disclosure does not necessitateany particular processing of the data in the sender 200 and that neitherthe SOVA type Viterbi decoder 261 nor the Reed Solomon decoder 283 ofthe receiver device 250 are modified for its implementation.

In one particular embodiment of the invention, a copy comprises a ReedSolomon code word. In a first variant, each couple contains only a partof a code word and, in this case, it is necessary to reconstitute thecode word from different copies before applying the algorithms of FIGS.6 to 8. In a second variant, a copy comprises several code words. Inthis case, the algorithm of FIGS. 6 to 8 must be applied successively tothe data constituting each code word.

FIG. 3 provides a schematic illustration of an architecture of atransmission system based on a spatial redundancy of the data accordingto a second particular embodiment of the invention.

The architecture of the sender device 200 is unchanged and shalltherefore not be described in detail again.

FIG. 3 illustrates the case of a receiver device 350 comprising two PHYmodules RX 260 that are identical to those described in FIG. 2 and oneMAC RX module 370.

Each PHY RX module 260 has an antenna 267 for receiving data coming fromthe sender device 200. Each PHY RX module 260 gives the MAC RX module270 pieces of corrected data and reliability tables coming from theradio signal received by its antenna.

In the example of FIG. 3, corrected pieces of data D′ accompanied byreliability tables Rel′ are (to the left of FIG. 3) on the links 271 and272 linking the first PHY RX module 260 and the MAC RX module 370.

Again, the corrected pieces of data D″ accompanied by reliability tablesRel″ are transmitted on links 273 and 274 connecting the second PHY RXmodule 260 (to the right in the FIG. 3) and the MAC RX module 370.

The pieces of data and information are received in the MAC RX module 37by two instantiations of a PHY/MAC input/output interface block 375which takes responsibility for the communication between the MAC RXmodule 370 and the two PHY modules RX 260. Once the data has been putinto a format enabling processing in the MAC RX module 370, these piecesof data are transmitted to two de-interleavers 381 which carry out anoperation in reverse to the one performed when sending by theinterleaver 231; these de-interleavers 381 also de-interlace theinformation on reliability that accompany the pieces of data coming fromthe SOVA type Viterbi decoders.

The difference in the implementation of the disclosure in onearchitecture of a transmission system based on a temporal redundancy(the case of FIG. 2) and the implementation in an architecture of atransmission system based on a spatial redundancy with two physicalmodules (the case of FIG. 3) is therefore limited to the duplicating ofthe PHY RX module 260, the PHY/MAC interface module 375 and thede-interleaver 381.

The algorithms implemented in the PHY multicopy modules 268 and the MACmulticopy modules 282 and 382 are identical for both types ofredundancy.

FIG. 4 illustrates a flowchart of an algorithm for generating areliability table implemented in a PHY RX module included in thereceiver device of FIG. 2 or 3, according to one particular embodimentof the invention.

This algorithm is implemented in each multicopy module PHY 268 of thereceiver device 250 and 350 and enables the creation of the reliabilitytables intended for the MAC multicopy module 282 and 382 integrated intothe MAC RX module 270 and 370.

Let us take a parameter S corresponding to a predetermined maximumnumber of erasures desired. This number is for example defined by theuser. Its value may take at most the value “n−k” for a Reed Solomon codeC(n,k).

This value of the parameter S can also be smaller than this maximumvalue so as to leave an error correction margin for the decoder.

For example, for a Reed Solomon code C(114, 96), the correction capacityof this code is 18 (114−96=18). Hence, the maximum value for theparameter S is 18. For example, S is taken to be equal to 10.

In a first step 410, the PHY multicopy module 268 gets reset and loadsthe parameter S, the set of data coming from the Viterbi decoder 261 aswell as the information on reliability pertaining to this data.

In a following step 420, an intermediate table is created bydetermining, on the basis of “bit” reliability information given by theViterbi decoder 261, the information on reliability of the “Reed Solomonsymbols”. These pieces of information on reliability are then classifiedin rising order of their level of reliability (it may be recalled thateach piece of information on reliability is associated with a level ofreliability).

In a following step 430, the reliability table 500 (FIG. 5) is createdout of the intermediate table as follows.

First of all, the indices are determined for a number N of symbols forwhich the reliability levels are the lowest, and therefore correspond tothe position indices situated at the head of the intermediate table.Then, these indices are inserted into the start of the reliabilitytable.

The number N corresponds to the number of pieces of reliabilityinformation transmitted by the PHY RX module 260 to the MAC RX module270 or 370. This number N depends on the rate of information that thecommunications link 272 or 273 can support.

The greater the value of N, the higher the performance levels ofdecoding of the system, but the costlier it is in terms of setting upthe hardware part (including the necessary bit rate between themodules).

The maximum value of N corresponds to the number of symbols contained inthe copy. In such a case, this amounts to doubling the bit rate of datato be transmitted between the PHY RX module 260 and the MAC RX module270 or 370. This is not desirable for high-bit-rate applications.

Then, on the N following positions of the reliability table, levels ofreliability are inserted corresponding to the indices inserted at the Nfirst positions of the reliability table.

Then, at the end of the reliability table, a level of overallreliability is inserted, this overall level being for example the meanvalue (“Relmean”) of the reliability levels of the symbols included inthe data block (i.e. the copy) coming from the Viterbi decoder 261.

In a following step 440, the reliability table formed by 2*N+1 values isconverted into a signal to be transmitted to the MAC RX module 270 and370 on the communications link 272 or 273. The nature of the conversiondepends on the chosen technology of data transfer between the PHY RXmodule 260 and the MAC RX module 270 or 370, for example a serializationor else again the application of a particular protocol. The technologyof transfer chosen is a function of the performance values desired andof the hardware constraints.

We shall now provide the detailed example of the computation of anadditional rate of information to be transmitted to the MAC RX module270 or 370 to implement an embodiment of the invention.

For a system implementing a Reed Solomon code C(114, 96) with ReedSolomon symbols encoded on eight bits, a value of N parametrized at 4,an index encoded on seven bits and a reliability value encoded on eightbits, the number of bits to be transmitted is 114*8=912 bits. Thereliability table has 4*(7+8)+8=68 bits. This gives an additional rateof information to be transmitted of 7%, this rate corresponding to theratio of the 68 bits of the reliability table on the 912 bitscorresponding to the data stream to be transmitted.

It can thus be seen that, in this example, the amount of data added toobtain an embodiment of the invention is small as compared with thenumber of pieces of data to be transmitted.

FIG. 5 presents an example of a reliability table generated byapplication of the flowchart of FIG. 4.

The first four values (Ind1 to Ind4) of the reliability table (with N=4)correspond to the position indices of the symbols having the lowestlevels of reliability (Ind1 being the index corresponding to the leastreliable piece of data, i.e. the lowest level of reliability) among thelevels of reliability of all the symbols of the associated copy.

The four values Re11 to Re14 correspond to the levels of reliability ofthe four corresponding symbols (i.e. the symbols for which the positionindices are Ind1 to Ind4).

The last value “Relmean” of the table corresponds to the overall levelof reliability (mean value computed on all the levels of reliability ofthe symbols of the associated copy).

FIG. 6 presents a first part of this overall flowchart in the generalcase where the number of received copies is greater than or equal totwo.

It must be noted that the flowchart of FIG. 6 is the same whatever theorigin of the multicopies (i.e. whether it pertains to a temporalredundancy or to a spatial redundancy).

Let m be the number of received copies, n the index of the symbolswithin each copy and i the index of the copies (1<i<m).

In the initializing step 610, the m copies as well as the m associatedreliability tables are received from the m PHY modules RX 260. Thevalues of an erasures vector EV(n) for each index n are also set at zeroin this step 610.

In a step 615, the index n is set at the value 1.

In a following step 620, a check is made to see if this index n islisted in all the received reliability tables.

If the result of the test of the step 620 is positive, it means that thelevel of reliability of the corresponding symbol is low on all thecopies. This symbol is then considered to be erased. In a step 625, anerasure is marked for this position index n. To this end, the elementEV(n) of the erasures vector of this index n is then set at a value “1”informing the Reed Solomon decoder that this symbol is erroneous. Thevalue of the symbol to be decoded So(n) for this index n can be set atany value since it will not be used by the decoder.

If the result of the test of the step 620 is negative then, in a step630, the symbols Si(n) for which the index is listed in the reliabilitytable of the copy i are eliminated. This operation therefore eliminatesthe symbols which have been detected as being among those least reliableby the Viterbi decoder.

In a following step 640, the integrity of the remaining symbols (i.e.the non-eliminated symbols) is checked by ascertaining that thesesymbols indexed n have the same value.

If the result of the test performed in the step 640 is positive, theprobability of having the same errors in each of the copies being verylow, it is deemed to be the case that the received value is good andthat the symbols are therefore reliable. The erasures vector EV(n)remains zero for this index n and, in a step 645, the symbol to bedecoded So(n) for this index n is assigned the value of the symbolSi(n).

It must be noted that the steps 640 and 645 are not indispensable forthe proper running of the algorithm of FIG. 6, given the following stepsin which checks are made to see if a majority of the remaining symbolspossesses a same value of symbols.

If the result of the test performed in the step 640 is negative, itmeans that one or more of the remaining symbols has been corruptedduring transmission although the computed reliability index indicatesthat the value of the symbols is reliable.

In this case, a check is made in a step 650 to see if a majority of thesymbols Si(n) remaining among the j (1<j≦i) symbols chosen in the step630 have the same value.

If the result of the test of the step 650 is positive, then a valueSk(n) received in a majority k of the copies is considered to be theright one. In a step 655, this value Sk(n) is then assigned to thesymbol to be decoded So(n) for this index n.

If the result of the test of the step 650 is negative, i.e. if there areas many symbols Si(n) having different values, then the valuetransmitted by the copy r having the highest overall reliability value“Relmean” is chosen.

In the step 660, the symbol to be decoded So(n) is then assigned thevalue of the symbol Sr(n) of the copy r.

At output of the steps 625, 645, 655 and 660, the value of the symbolSo(n) has been assigned whatever the values of the copies. The levels ofreliability of the symbols as well as the erasures vector EV(n) havebeen possibly updated.

In a step 670, the operation passes to the next position index byincrementing the value by n.

In a following step 680, a check is made to see if the end of the copyhas been reached.

If the result of the test of the step 680 is negative, the step 620 isagain executed to process the next symbol.

If not, the operation passes to the second part of the overall algorithmdescribed here below with reference to FIG. 8, used especially to managethe number of erasures.

FIG. 7 presents a variant of the above-mentioned first part in theparticular case where the number of copies received is equal to two.

It must be noted that the flowchart of FIG. 7 is the same whatever theorigin of the multicopies, i.e. that this is a temporal redundancy or aspatial redundancy.

In an initialization step 710, the two copies as well as the twoassociated reliability tables coming from the two PHY modules RX 260 arereceived. This step sets the values of the erasures vector EV(n) at zerofor each index n.

In a step 715, the index n is set at the value 1.

In a following step 720, a check is made to see if this index n islisted in the two reliability tables.

If the result of the test of the step 720 is positive, it means that thelevel of reliability of the corresponding symbol is low on both copies.This symbol is then considered to erased.

In a step 725, an erasure is marked for this position index n. To thisend, the element EV(n) of the erasures vector of this index n is thenset at a value “1” informing the Reed Solomon decoder that this symbolis erroneous. The value of the symbol to be decoded So(n) for this indexn can be set at any unspecified value because it will not be used by thedecoder.

If the result of the test of the step 720 is negative, then in a step730 a check is made to see if the index n is listed in the reliabilitytable of the first copy named TF1.

If the result of the step 730 is positive, the index n being not listedin all the tables, it means that in principle the symbol of the secondcopy is reliable.

In a step 735, the value S2(n) is then assigned to the symbol to bedecoded So(n) for this index n.

If the result of the step 730 is negative then, in a step 740, a checkis made to see if the index n is listed in the reliability table of thesecond copy, named TF2.

If the result of the step 740 is positive and since the index n is notlisted in the reliability table of the first copy TF1, it means that inprinciple the symbol of the first copy is reliable.

In a step 745, the value S1(n) is then assigned to the symbol to bedecoded So(n) for this index n.

If the result of the step 740 is negative (i.e. if the index n is notlisted in any of the two reliability tables), then in a step 750 a checkis made to test the integrity of the symbols S1(n) and S2(n) in checkingto see if these symbols indexed n have the same value.

If the result of the test performed in the step 750 is positive, theprobability of having the same errors on each of the copies being verylow, the received value is considered to be the right value. Thesesymbols are therefore reliable and their value can be transmitted to theReed Solomon decoder in a step 755. The erasures vector remains zero forthis index.

If the result of the test performed in the step 750 is negative, itmeans that one or both symbols has (have) been corrupted duringtransmission, even though the reliability index or indices computedindicate that the value or values of the symbol or symbols is/arereliable.

In this case, in a step 760, the value transmitted by the copy r whichwill have the highest overall value of “Relmean” will be chosen. Thesymbol to be decoded So(n) is then assigned the value of the symbolSr(n).

It must be noted that the step 760 can also be performed directly afterthe executions of the steps 720 to 740 without performing the test ofthe step 750. In this case, for each received copy whose given positionindex symbol n is a symbol, called a remaining symbol, such that thegiven position index n is not listed in the reliability table of thefirst copy or in the reliability table of the second copy, the step 760comprises two operations. Firstly, it is a step for obtaining a overallreliable level “Relmean” associated with said received copy. Secondly itis a step for choosing, for said symbol to be decoded (So(n)) having agiven position index (n) in said copy to be decoded, the value of theremaining symbol included in the received copy associated with thegreatest level of overall reliability obtained.

It is important to note that, in this case, the risk of a wrong choiceof symbol is notnil. In this particular embodiment of the invention, itis then advantageous not to choose the maximum value for the parameter Sand to thus keep an error correction capacity in the Reed Solomondecoder. In other words, a value of S smaller than the correctioncapacity C of the Reed Solomon decoder is chosen.

At the exit from the steps 725, 735, 745, 755 and 760, the value of thesymbol So(n) that has been assigned whatever the values of the copiesand the reliability of the symbols, the erasures vector EV(n) havingbeen possibly updated, the next position index is tested by incrementingthe value of n in a step 770.

In a following step 780, a check is made to see if the end of the copyhas not been reached.

If the result of the test of the step 780 is negative, the step 720 isagain executed to process the following symbol.

If not, the operation passes to the second part of the overall algorithmdescribed here below with reference to FIG. 8, enabling management ofthe number of erasures.

The number of erasures determined by the first part of the overallflowchart presented in FIG. 6 or FIG. 7 can be greater than the desirednumber of erasures S, which is very detrimental when the parameter is Sis fixed at its maximum value (n−k). Indeed, the Reed Solomon decoder inthis case gives wrong data. The second part of the total flowchartillustrated in FIG. 8 proposes a solution to this problem.

FIG. 8 shows a second part of the overall flowchart making it possibleespecially to manage the number of erasures.

A step 810 is used to compute the determined number of erasuresdetermined by the first part of the overall flowchart of FIG. 6 or 7, intaking, for example, the sum of the elements of the erasures vector EV(the indices of the symbols considered to be erased are at one and theothers at zero).

Then, in this same step 810, a check is made to see if this number oferasures is smaller than the predetermined maximum number of erasures S.

If the result of the test of the step 810 is positive, the erasuresvector is considered to be valid and the operation passes directly tothe step 860 in which the copy to be decoded (the symbol block) and theerasures vector as determined by the algorithm of FIG. 6 or 7 aretransmitted to the Reed Solomon decoder.

If the result of the test of the step 810 is negative, the erasuresvector and the copy to be decoded (symbol block) are re-defined in thesteps 820 to 850.

In a step 820, all the elements of the erasures vector are reset atzero.

In a step 830, a work table TEV (refer to FIG. 10 for example) formed bythe index of low reliability symbols common to all the reliabilitytables is created, these indices corresponding to a positive test duringthe step 620 (FIG. 6) or 720 (FIG. 7). In the work table TEV, each ofthese indices has, associated with it, the highest level of reliabilityamong all the reliability levels associated with this index in all thereliability tables.

Then, these indices are classified by rising values of reliabilitylevels.

In a step 840, then the S values of the elements of the erasures vectorswhich are the S first indices of the table are positioned at the value“1”. The symbols which are these S indices will be considered to beerased and their value therefore has no importance.

In a step 850, the values So(n) for which the index n is not part of theS indices referred to here above receive the value Si(n) possessing thebest reliability level.

All the elements of the erasures vector and symbols of the copy to bedecoded (symbol block) having been determined, these pieces of data arethen sent to the Reed Solomon decoder in a step 860.

FIGS. 9 a to 9 c illustrate an example of application of the overallflowchart of FIGS. 6 to 8 in the case of a system with two copies, andwhen there is little correlation between the two copies.

In this case, the errors due to the transmission of each copy do notaffect the same symbols and the number of erasures created remains belowthe number S of erasures desired.

In this example, the value of the parameter S is fixed at 3 and thenumber of pieces of reliability information transmitted by the PHY RXmodule 260 to the MAC RX module 270 or 370 is fixed at 4. It is alsoassumed that there are no errors in the symbols not listed in thereliability table.

FIG. 9 a provides a schematic illustration of an example of areliability table 901 received for the first copy 902. Among the 114symbols that form the first copy, the symbols 7, 59, 103, 110respectively have reliability levels −20, 10, 25, 28. The level ofoverall reliability of the set of symbols of the first copy is 39.

FIG. 9 b is a schematic illustration of an example of a reliabilitytable 903 received for the second copy 904. Among the 114 symbols thatform the second copy, the symbols 14, 75, 103, 107 respectively havereliability levels of −40, 26, 25, 32. The level of overall reliabilityof the set of symbols of the second copy is 44.

FIG. 9 c is a schematic illustration of the result of the application ofthe overall algorithm of FIGS. 7 and 8.

The first row 905 gives the index of the symbols in the copy. The secondrow 906 gives the source (C1 for the first copy and C2 for the secondcopy) of the symbols forming the copy to be decoded, i.e. the copy to besent to the decoder. For the symbols 14, 75 and 107, the symbols arethose coming from the first copy. For the symbols 7, 59 and 110, thesymbols are those coming from the second copy. Since all the othersymbols have the same value, any one of the two copies can be selected.The value of the symbol 103, denoted E, has no importance since it hasbeen declared to be efficient (in the presence of an erasure).

The third row 907 gives the contents of the erasures vector. Only theindex element 103 is at one since this is a symbol of low reliability onboth copies. The other elements remain at zero. There is therefore onlyone erasure declared to the decoder.

FIGS. 10 a to 10 d illustrate the example of application of the overallflowchart of FIGS. 6 to 8 in the case of a system with two copies andwhen there is much correlation between the two copies.

In this case, the errors due to transmission of each copy affect thesame symbols and the number of created erasures between two shouldtherefore be limited so as not to exceed the desired number of erasuresS.

In this example, the parameter S is fixed at 3 and the number of piecesof information on reliability transmitted by the PHY RX module 260 andto the MAC RX module 270 or 370 is set at 4.

The assumption is also made wherein there is no error in the symbols notlisted in the table of reliability.

FIG. 10 a is table of reliability 1001 received for the first copy.Among the 114 symbols which form the first copy, the symbols 7, 59, 103,110 respectively have levels of reliability of −20, 10, 25, 28. Thelevel of overall reliability of the set of symbols of the first copy is39.

FIG. 10 b is table of reliability 1003 received for the second copy.Among the 114 symbols which form the second copy, the symbols 7, 59,103, 110 respectively have levels of reliability of −40, 26, 25, 32. Thelevel of overall reliability of the set of symbols of the second copy is44.

FIG. 10 c illustrates an example, on the basis of FIGS. 10 a and 10 b,of a table TEV 1008 created in the step 830 of the flowchart of FIG. 8.After classification in a rising order of levels of reliability, thefirst four elements of this table TEV are the indices of the leastreliable symbols on the two copies. The following four elements are, foreach index, the best values of reliability amongst the two received. Themaximum number of erasures desired being set at 3, only the symbolscorresponding to the first three indices of the table TEV (7, 103, 59)are considered to be erased (hence the element corresponding to thisindex is set at 1 in the erasures vector). For the fourth index 110listed in this table, the symbol transmitted to the decoder is the mostreliable of the two received. In this example, this is the symbol comingfrom the second copy.

FIG. 10 d illustrates the final result of the application of the overallalgorithm of FIGS. 7 and 8.

The first row 1005 gives the index of the symbols in the copy. Thesecond row 1006 gives the source of the symbols which constitute thecopy to be decoded (i.e. the copy to be sent to the decoder). For theindex 110, the symbol is the one coming from the second copy. For theindices 7, 59, 103, the value E has little importance since thesesymbols are declared to be erased. Since all the other symbols have thesame value, any unspecified one of the two copies is selected.

The third row 1007 gives the content of the erasures vector. The indicesof 78, 59, 103 are at 1, while the others remain at 0. There aretherefore three erasures declared to the decoder.

The invention claimed is:
 1. Method for determining a copy to be decodedand an associated erasures vector by means of a receiver devicereceiving at least two copies of a same piece of data encoded accordingto an error correction code, each received copy comprising severalsymbols, each associated with a position index in said copy, said copyto be decoded being designed to be decoded in taking account of theerasures vector by means of a decoder of a first type applying adecoding adapted to the error correction code, said method wherein itcomprises steps for: obtaining, for at least one symbol of each copyreceived, a level of reliability of transmission associated with saidsymbol; marking or not marking, for at least one given position indexassociated with a symbol included in said copy to be decoded, an erasurefor said given position index in said erasures vector as a function ofthe reliability levels associated with the symbols which are positionedat said given position index in said received copies.
 2. Methodaccording to claim 1, wherein it comprises steps for: obtaining, foreach received copy, a set of a number N of position indices associatedwith N symbol(s) of said received copy for which the associated level orlevels of reliability is/are the N lowest level or levels from amongstthe levels of reliability of all the symbols of each copy received, withN being equal to or greater than 1; marking, for at least one givenposition index corresponding to a symbol included in said copy to bedecoded, an erasure in said erasures vector if said given position indexbelongs to all the sets of N position indices obtained for all thereceived copies.
 3. Method according to claim 2, wherein the number N ofposition indices of each set is a function of a maximum information bitrate that can be accepted by an interface from: a first processing blockcomprising a decoder of a second type generating said levels ofreliability, a second processing block including said first type ofdecoder.
 4. Method according to claim 2, wherein it comprises stepswhich, for a symbol positioned at a given position index in said copy tobe decoded, are steps for: determining a set of a remaining symbol orsymbols such that, for each copy received, the symbol positioned at thegiven position index in said received copy belongs to said set if saidgiven position index does not belong to the set of N position indicesobtained for said received copy; checking to see if there is a majorityof remaining symbols possessing a same value; in the event of a positivecheck, choosing said same value for said symbol positioned at said givenposition index in said copy to be decoded.
 5. Method according to claim4, wherein it comprises steps: in the event of negative check: forobtaining, for each copy received whose symbol positioned at said givenposition index is a remaining symbol, a level of overall reliabilityassociated with said copy received; for choosing, for each symbolpositioned at said given position index in said copy to be decoded, thevalue of the remaining symbol which is included in the received copyassociated with the greatest level of overall reliability obtained. 6.Method according to claim 2, said receiver device receiving said firstand second copies, wherein it comprises steps which, for at least onegiven index position, corresponding to a symbol included in said copy tobe decoded, are steps for: fiabilité globale selecting, for said symbolpositioned at said given position index in said copy to be decoded, thevalue of the symbol at said given position index in the second copyreceived if said given position index belongs to a first set of Nposition indices obtained for said first copy received; selecting, forsaid symbol positioned at said given position index in said copy to bedecoded, the value of the symbol at said given position index in thefirst copy received if said given position index belongs to a second setof N position indices obtained for said second copy received.
 7. Methodaccording to claim 6, wherein it comprises a step which, if said givenposition index belongs to no set amongst said first and second sets of Nposition indices, and if the symbols in the first and second copiesreceived possess a same value, is a step for choosing said same value.8. Method according to claim 6, wherein it furthermore comprises stepswhich, if said given position index belongs to no set amongst said firstand second sets of equal to one or more position indices, are steps for:obtaining, for each received copy whose symbol positioned at said givenposition index is a remaining symbol such that the given position indexdoes not belong to the set of N position indices obtained for eachreceived copy, a level of overall reliability associated with saidreceived copy; choosing, for said symbol positioned at said givenposition index in said copy to be decoded, the value of the remainingsymbol which is included in the received copy associated with thegreatest level of overall reliability obtained.
 9. Method according toclaim 1, wherein it comprises steps for: checking to see whether a totalnumber of one or more erasures marked in said erasures vector is greaterthan a maximum predetermined number S; and in the event of a positiveverification: resetting the erasures vector in eliminating thepre-marked erasure(s); building a list of position indices for which anerasure has been pre-marked in the erasures vector; associating, witheach position index of the list, the highest level of reliabilityamongst those associated with the symbols positioned at said positionindex in the received copies; classifying the list by rising order oflevels of reliability associated with the position indices; marking anerasure for the S first position indices of the classified list;choosing, for each other position index of the classified list notmarked as an erasure, for the symbol positioned at said position indexin said copy to be decoded, the value of the symbol for which the levelof reliability is the highest amongst the levels of reliability of thesymbols positioned at said position index in the received copies. 10.Method according to claim 9, wherein said predetermined maximum number Sof erasures is smaller than or equal to a limit capacity of correctionof said error correction code.
 11. Method according to claim 1, whereinthe decoder of the first type is a block decoder and the decoder ordecoders of the second type i/are a convolutive decoder or convolutivedecoders.
 12. Computer-readable storage means storing a computer programcomprising a set of instructions executable by a computer forimplementing a method for determining a copy to be decoded and anassociated erasures vector by means of a receiver device receiving atleast two copies of a same piece of data encoded according to an errorcorrection code, each received copy comprising several symbols, eachassociated with a position index in said copy, said copy to be decodedbeing designed to be decoded in taking account of the erasures vector bymeans of a decoder of a first type applying a decoding adapted to theerror correction code, said method wherein it comprises steps for:obtaining, for at least one symbol of each copy received, a level ofreliability of transmission associated with said symbol; marking or notmarking, for at least one given position index associated with a symbolincluded in said copy to be decoded, an erasure for said given positionindex in said erasures vector as a function of the reliability levelsassociated with the symbols which are positioned at said given positionindex in said received copies.
 13. Receiver device adapted todetermining a copy to be decoded and an associated erasures vector, saidreceiver device receiving at least two copies of a same piece of dataencoded according to an error correction code, each received copycomprising several symbols, each associated with a position index insaid copy, said copy to be decoded being designed to be decoded intaking account of the erasures vector by means of a decoder of a firsttype applying a decoding adapted to said error correction code. saidreceiver device wherein it comprises: means for obtaining, for at leastone symbol of each copy received, a level of reliability of transmissionassociated with said symbol; means for marking, for at least one givenposition index associated with a symbol included in said copy to bedecoded, an erasure for said given position index in said erasuresvector as a function of the reliability levels associated with thesymbols which are positioned at said given position index in saidreceived copies.